Heat pumps are used for both heating and cooling. One of the purposes of this invention is to provide, in a dual system having both a heat pump and a fossil fuel furnace, greater comfort when the heat pump is used for heating. It does so by allowing the user to program various times of the day and night when the heat pump will be utilized for space heating and other times when the back-up fossil fuel furnace will carry the heating load. The modes of operation during these programmed times are also dependent on the temperature of the outside ambient air.
Conventional fossil fuel furnaces (powered by gas or oil) deliver air from the room registers at a temperature in the range of 100.degree. to 140.degree. F. Heat pumps, on the other hand, produce heated air at 85.degree. to 95.degree. F. Since the human body has a natural temperature of about 98.6.degree. F., air below that temperature feels cool and chilly. This fact is compounded by the need to circulate a greater quantity of air to harvest the lower level of heat generated by the heat pump. When air is cooler to begin with, the increased air flow makes it feel drafty and even more unpleasant.
To overcome this problem, the heat pump should be used only at times when the residents are least likely to object to "cool" heat. These periods may occur (1) when no one is home, (2) when everyone is asleep, or (3) when outdoor temperatures are mild. This is accomplished preferably by means of a programmable device that modifies thermostat function. The device of this invention accomplishes these goals without causing disruption to normal thermostat operation. Should there be a need, the device can also be manually defeated at will by the user.
The control of this invention may be programmed to perform in the following manner:
Two temperature limits are identified. The lower one is that below which the heat pump is not able to sustain the heating demand of the building at the designated outdoor temperature. This designated outdoor temperature may be called the "Balance Point Temperature". The other temperature limit is higher and is chosen such that the heat pump supplies heated air considered acceptable to the occupants when the outdoor air temperature is at this designated value or higher. This designated outdoor air temperature may be called the "Comfort Temperature". Examples of these temperature limits are 32.degree. F. and 45.degree. F., respectively. The selection is dependent on the heat pump and fossil fuel furnace used, the building heat loss and individual personal preferences.
According to the theory of operation, the fossil fuel furnace is automatically selected by the control unit to supply all of the building heat when the outdoor temperature is below the Balance Point Temperature. Above the outdoor Comfort Temperature, all heating is generated by the heat pump. In the temperature range between the two limits, sometimes called the economy range, the control unit selects one heating unit or the other depending on the times of the day selected by the user. During early morning rising times when many people feel chilled, the user will probably program the unit to heat with the fossil fuel furnace so that the air feels warm and comfortable. During other daytime hours when there is more physical activity or when the residents are away at work or school, the heat pump can deliver its "cooler" warm air. During the evening hours, when activity levels may be reduced, the fossil fuel furnace might be selected again. To complete the daily cycle, the heat pump would likely be chosen for heating during the night time sleep period.
It is a primary object of this invention to provide a control for the operation of separate heating units such as a conventional fossil fuel furnace and a heat pump having the foregoing features.
A further object is to provide a control for the selective operation of either a fossil fuel furnace or a heat pump which is simple and easy to operate, composed of a relatively few simple parts, and capable of being readily and inexpensively manufactured.
Other objects, features and advantages of the invention will become more apparent as the following description proceeds, especially when considered with the accompanying drawings.